This invention relates in general to the field of power converters, and, more particularly, to diode-clamped series resonant power converters.
In conventional pulse width modulated (PWM) power converters, switching losses increase as frequency increases. Efficiency therefore decreases as the frequency increases and the benefits of smaller components cannot be fully realized.
Resonant power converters process power in sinusoidal waveforms and switching losses can be reduced by zero current switching (ZCS) or zero voltage switching (ZVS). Either approach, however, is associated with the disadvantages of high peak currents or voltages and wide frequency range for control.
The diode-clamped series resonant converter has been proposed to reduce the problem of high component stresses, but does not address the problems associated with frequency control. As an example, consider a clamped series resonant converter (CSRC). For design parameters of input voltage range of 22 to 50 Volts DC (VDC), output voltage of 5 VDC, output current of 3 Amperes (A) and output power of 15 Watts (W), the peak currents may vary from approximately 4 A to more than 15 A, and the corresponding resonant frequencies of the power converter range from about 600 (kHz) to almost 97 kHz. If the output power decreases from 15 W, the switching frequency must decrease to maintain constant output voltage.
Thus, even with zero current switching in the diode-clamped series resonant converter, the advantage of the resonant converter for high frequency operation due to the reduction of switching losses largely disappears due to the frequency control problem. Filter components and transformers must be large enough to accommodate the lowest switching frequency. In addition, as in the above example, the still relatively high peak current gives rise to large component stresses. Two other disadvantages of the CSRC converter are: 1) electromagnetic interference is generated at the low frequencies required to maintain regulation; and, 2) the output filter capacitor must be large to maintain low output voltage ripple.
A constant frequency resonant power converter significantly reduces the above disadvantages. Several complex schemes have been proposed for achieving a resonant power converter capable of constant frequency operation. One scheme involves the use of interconnected multi-resonant converters. Multi-resonant converters involve at least two resonant converters in a complex interconnection employing active switching. Additional switches or more complex switching arrangements are required for the multi-resonant converters than for the constant frequency CSRC: e.g., for a half-bridge configuration either two additional switches on the secondary side, one additional switch on the secondary side, or one bidirectional switch on the primary side of the transformer are required.
Another proposed method for achieving a constant frequency resonant power converter uses a variable capacitance device in the output filter of a converter to reduce the output ripple voltage. The variable capacitance device requires an independent control voltage to vary the capacitance, however, and in such an arrangement it is preferable that the capacitance not be under the influence of the input source voltage.
Neither multi-resonant converters nor variable capacitance converters employ the switching of a resonant capacitive or inductive element during the power transfer cycle in the manner advocated here. The present method uses a feedback control system to generate a pulse-width modulated signal to switch the resonant capacitive or inductive element at the appropriate time.
It is highly desirable to provide for a resonant power converter capable of constant frequency operation with ZCS or ZVS while reducing peak current and voltage. It is particularly desirable to provide for the use of smaller magnetic devices and smaller filter capacitors than with variable frequency power converters, in a constant frequency power converter exhibiting reduced electromagnetic interference.